In the prior art, there exists a multitude of active matrix circuits for OLED-displays having at least two transistors for each organic light emitting diode, wherein the transistors may be of the same or of a different type (NMOS and PMOS).
Prior art FIG. 1(a) shows a common active matrix circuit for an organic light emitting diode according to the state of the art. Every pixel circuit can have two NMOS transistors T1 (102) and T2 (103), the gate of transistor T1 being connected to the scan-line (105) and the drain of transistor T1 being connected to the data-line (106). The source of T1 is connected to the gate of transistor T2. The capacitor C1 (104) is connected between the gate and the source of T2. Such an active matrix circuit plus the organic light emitting diode (101) is called as AMOLED pixel in this invention. The AMOLED display in FIG. 1(a) (111) can have three rows and three columns and in total of 9 AMOLED pixels.
When the scan-line is activated (High), transistor T1 is switched on. Then, the driving transistor T2 receives the signal from the data-line and an electric current may flow from the voltage source Vs (108) via the column traces through the organic light-emitting diode to the ground, as indicated by the bold line in FIG. 1(a). In this description the traces from the positive pole of the power supply (voltage source) to the AMOLED pixels (anode) are called as column line (109). The power lines at the opposite side, not explicitly drawn in FIG. 1(a), namely the traces from the negative pole (ground) of the power supply to the AMOLED pixels (cathode) are called ground line (110). The data signal is an analog signal, i.e. not a high or a low signal but somewhere in between. The level of the signal depends on the desired luminance of the organic light-emitting diode. A higher luminance requires a higher diode current. When the desired gate voltage of transistor T2 has been applied, the scan signal of the selected row may be deactivated in order to select another row of the display. The capacity C1 is needed to preserve the gate voltage of transistor T2, permitting the electric current to flow constantly through the diode in the desired strength.
As transistor T2 in this circuit is always operated in the saturation region as an electric current source, a very precise and stable threshold voltage is required. But if the active matrix circuit is to be manufactured using a low cost process, transistors may exhibit large variations in their threshold voltage, that may also drift with time. Moreover, the circuit may only be operated at a high power loss, because a substantial voltage drop at the driving transistor T2 is needed for the current source mode. So the power supplied by the voltage source Vs (FIG. 1(a)) has to be considerably higher than the forward voltage of an OLED diode. With this drive scheme, pixels are illuminated substantially continuously.
This drive scheme, however, is disadvantageous because it is not power efficient and requires a complex and expensive manufacturing process for the active matrix. Also complex pixel circuits e.g. with more than two transistors are needed to compensate the variation and drift of the threshold voltage of the driving transistor. A large active-matrix OLED-display is therefore much more expensive than an active-matrix LCD-display. Consequently, large active-matrix OLED-displays may still not compete with corresponding LCD-displays.